This invention relates generally to vortex-type flowmeters, and more particularly to an isolated two-wire signal transmission system for such meters whereby the two wires serve to conduct d-c for powering an amplifier associated with the meter as well as the a-c signal yielded by the amplifier.
A vortex-type flowmeter is adapted to measure the flow rate of a fluid passing through a conduit by producing fluidic pulses or oscillations whose repetition rate or frequency varies in accordance with flow rate. Two species of vortex-type meters are commercially available in the United States, one being called the so-called "Swirlmeter," and the other the bluff-body vortex-shedding type.
In Swirlmeters, such as those described in U.S. Pat. Nos. 3,279,251; 3,314,289 and Re 26,410, a homogeneous fluid whose flow rate is to be measured is forced to assume a swirl component. This is accomplished by feeding the fluid into the inlet section of a flow tube having a fixed set of swirl blades therein which imparts a swirling motion to the fluid passing therethrough. Precession takes place about the central axis of the flow tube at a discrete frequency that is a function of the volumetric flow rate. Cyclic variations in local fluid velocity occurring by reason of precession are detected by a sensor to provide electrical pulses whose frequency is measured to provide an indication of flow rate.
IN the bluff-body type of vortex meter, such as those described in U.S. Pat. Nos. 3,116,639; 3,587,312 and 3,854,334, an obstacle is mounted within the flow conduit transversely with respect to the flow axis to create a Karman vortex street, the frequency of whose vortices is proportional to flow rate. These are sensed to produce a signal whose frequency is indicative of flow rate.
One form of sensor commonly used in commercially-available Swirlmeters or vortex-shedding flowmeters is the piezoelectric quartz pressure transducer. This transducer consists basically of three parts; namely, a metal casing which serves for mounting and to hermetically enclose the quartz elements, quartz elements yielding an electrical signal proportional to the applied pressure, and a diaphragm welded to the casing and transmitting to the quartz elements the pressure exerted by the medium being sensed.
One or a pair of such pressure sensors are used to sense the fluidic oscillations in the vortex meter. In order to simplify the wiring and to improve the reliability of the system, the casing of the sensor which is attached to the meter body is used as one of the transducer leads, the other lead being connected to the quartz elements.
Since the level of signal output from a quartz pressure sensor lies in the millivolt range (at low flow rates, this level drops to as low as one millivolt), local pre-amplification at the flowmeter site is desirable to obtain a good signal-to-noise ratio.
In a typical field installation for an industrial process system, there are usually several vortex-type flowmeters and associated pre-amplifiers together with various other instruments, all of these devices being tied together into one network. If in this network all flowmeters are electrically grounded at their own bodies, the resultant multiplicity of grounds would create ground loops and give rise to an intolerable level of electrical noise. It is essential, therefore, to provide electrical isolation between the various meters.
In order to effect such isolation, it has heretofore been the practice to supply power to the pre-amplifier associated with the meter by means of a separate isolated DC power supply or by way of a DC-to-DC converter, two wires being necessary for the transmission of d-c power. And in order to isolate the a-c signal output of the pre-amplifier, use is made of an optical coupler or of a transformer. Here too, for a-c signal transmission, two wires are required. While a transformer is preferable from the standpoint of power efficiency, because the a-c signal from the vortex meter lies in the very low-frequency range, a transformer appropriate to this range is necessarily large and quite expensive.
Inasmuch as in these prior transmission systems, the d-c power and a-c signal are conveyed on separate lines, three or four leads are required for this purpose, depending on the arrangement. Where the installation calls for an intrinsically-safe system, the existence of three or four leads per meter dictates a multiplicity of barriers or one very complex barrier. Thus existing wire transmission systems for conveying signal and power for a vortex meter are relatively complex and costly, particularly where intrinsic safety is required.